1. Field of the Invention
The present invention relates to a digital watermark technique for digital data, particularly image data.
2. Description of Related Art
Recently, from the viewpoint of copyright protection f or digital data such as image data, etc., much attention is being paid to a digital watermark technique. The digital watermark technique is defined as a technique of embedding specific information into digital data according to a predetermined rule so that the specific information cannot be extracted from the digital data without using at least the predetermined rule. According to this technique, for example, information on a purchaser of image data, etc. is beforehand embedded in the image data itself according to the predetermined rule in such a manner that the information is not visible, and when the image data are illegally copied, the embedded information is extracted according to the predetermined rule from the data thus illegally copied to specify a person (purchaser) who illegally copies the data.
FIG. 18 is a diagram showing the principle of information embedding/extracting processing into/from image data on the basis of a conventional digital watermark technique.
With respect to the information embedding processing, as shown in FIG. 18, when a bit bi (0≦i≦n) of bits b1 to bn constituting information to be embedded is equal to 1, the bright of each of pixel data located at predetermined positions 1 to m of the image data is increased by U, and when the bit bi is equal to zero, the brightness of each pixel data is reduced by U. This processing is carried out on all the bits b1 to bn while the embedding position is shifted, thereby embedding the digital watermark information in the image data.
With respect to the processing of extracting the digital watermark information thus embedded, for the bit bi (0≦i≦n) of bits b1 to bn constituting the information thus embedded, the brightness sum S of all the pixel data located at the predetermined positions 1 to m of the image data and the brightness sum R (Reference value) of all the pixel data located adjacently to the predetermined positions 1 to m are detected. If S−R≧T (here, T is varied in accordance with a demanded error rate, and it is set to satisfy T≧U×(the number m of pixel data in which bi is embedded)), it is judged that bi is equal to 1. If S−R≦−T, it is judged that bi is equal to zero. If −T<S−R<T, it is judged that no information is embedded in each of the pixel data located at the predetermined positions 1 to m. This processing is carried out on all the bits b1 to bn to extract the digital watermark information in the image data.
The digital watermark technique is described in more detail in Kobayashi, et al.: Data Hiding Based on Neighbor Pixels Statistics, In Proc. of IPSJ 56th Annual Conference, 1v-03, (1998).
When a geometrical deformation such as scale-up/scale-down or rotation is applied to image data in which digital watermark information as described above is embedded, the embedding positions of the bit data constituting the digital watermark information (the positions on the X-Y coordinate used when the bit data are embedded) are also deformed. Therefore, the digital watermark information cannot be extracted. A technique to overcome the above problem is disclosed in U.S. Pat. No. 5,636,292.
According to this technique, as shown in FIG. 19, a calibrating digital watermark for estimating what geometrical deformation is applied to the image data, that is, what scale-up/scale-down or rotation processing is applied to the image data in addition to the digital watermark information such as copyright information or the like, is beforehand embedded in the image data concerned. When the digital watermark information is extracted, the calibrating digital watermark information is extracted from the image data, and the degree of the deformation is analyzed, thereby estimating the scale-up/scale-down degree or the rotational angle applied to the image data concerned. Subsequently, the image data concerned are returned to the original image data (the image data before the geometrical deformation is applied) by referring to the scale-up/scale-up rate or the rotational angle thus estimated. Thereafter, the pixel data located at a predetermined position and the adjacent pixel data are extracted, and the differential value (S−R) therebetween is compared with a predetermined threshold value to extract the digital watermark information.